No country can rival America's success in assimilating immigrants. From its inception, the United States has received wave upon wave of newcomers. And despite the staggering numbers, it has done an incredible thing. It has turned not just the distant descendants of those millions of migrants, but also their grandchildren and even their children, into ordinary Americans.

In Europe, no country has come close to doing that—even though, as you might think, the smaller number of immigrants ought to have made the task easier. This is a huge difference, and one that now matters for a new reason. To see why, look at Britain. With renewed urgency after last month's bombings in London, the United Kingdom is grappling with domestic Islamic terrorism. The issue of unassimilated immigrants compounds and complicates that problem in ways that Americans, with their very different historical experience, may find difficult to understand.

For years, mainstream British politics has preferred to ignore the issues posed by the country's unassimilated Muslim minority. As in other parts of Europe, efforts to inject those questions into organized politics were apt to be dismissed (often with good reason) as bigotry. Now, for the first time, the issue refuses to be ignored. It has moved to the center of national politics.

Last week, in response to the July attacks on London, Prime Minister Tony Blair announced plans for harsh new anti-terror laws. These would include powers to close mosques where "extremist" ideas are being preached; to ban some Islamic organizations; and, contravening the European Convention on Human Rights, which Britain has adopted, to deport some foreigners to countries where the accused may be tortured or executed (although the governments concerned would have to promise to spare the deportees such measures). Grounds for deportation, Blair said, would include "fostering hatred, advocating violence to further a person's beliefs, or justifying or validating such violence." Such acts, of course, are not crimes under British law; if they were, the accused could be put on trial, rather than removed from the country by administrative action.

Most Britons, it seemed, believe that the new policies are justified. Quite possibly, many think them overdue. But there lies the problem. British Muslims (with the backing of civil-liberties groups and the Liberal Democrats, the country's third-largest political party) disagree: They see the new policies not merely as infringing on Muslims' civil rights but as an attack on the religion itself. The government replies that the new powers are directed chiefly at noncitizens, but Britain's Muslims are unimpressed by that argument. A Muslim preacher, facing deportation because the government deems his sermons to be inflammatory, may be a citizen of Pakistan, say, but he is still a Muslim—a brother in the faith.

These new plans of the British government are not the first of their kind. They follow a series of security laws enacted after 9/11 and some other proposals previously announced but not yet implemented. This profound shift in the law, even before the further curbs now being planned, had already prompted one of Britain's most senior judges to say that the greatest danger facing Britain was not terrorism, but the erosion of the country's traditional freedoms. Unlike in America, these liberties are not protected by a written constitution. A government like Blair's with a secure parliamentary majority can trample them with impunity, so long as it has at least the tacit support of the public. And Blair does.

Today, as you might expect, he has even stronger support than before. Until last month, many of the government's critics had accused it of scaremongering—of exaggerating the threat from terrorists in order to advance some pre-existing plan to curtail liberties—just as George W. Bush's critics have accused him. To the non-paranoid, that always looked like a phony charge. Regardless of whether it was a plausible or implausible accusation, however, it was certainly a stupid one, because it required only a successful terrorist attack, like the one in London on July 7, to refute it. As Blair rightly said last week, things have changed: Nobody is talking about scaremongering any more.

But this does not mean that Britain is united in purpose after the bombings. If the country had that kind of solidarity, it would be having a different kind of debate at the moment. A nation that is broadly united against the threat of terrorism will certainly want to argue about how far, if at all, valued civil liberties must be surrendered in exchange for greater security—and indeed whether any particular curtailment of freedom will or will not make people any safer. The United States is in a position to have that debate, and is having it (admittedly, not always in a very even-tempered fashion). Britain is different.

Because of its largely unassimilated Muslim population, the country is divided in a way that America is not. The July bombers were not foreigners who had sneaked in from abroad. They sprang from local Muslim communities. Not only that, but many of them gave every appearance of being good conscientious Muslims. The country is asking: How could that have happened? How many more of these people might there be—and what is to be done about it? Meanwhile, Britain is arguing not about what price in surrendered liberty all citizens should willingly pay to feel safer, as one would wish, but about whether Muslims are being collectively punished for the crimes of a handful of criminals.

A great underlying problem is that so many British Muslims feel little sense of community and solidarity with their fellow Britons, and vice versa. Whose fault is that? No doubt, there is plenty of blame to go around. Britain's Muslim immigrants and their children, like visible minorities in many countries, have to face outright racism and milder forms of prejudice, to be sure. They are discriminated against at work, no doubt, and in other ways are often made to feel excluded rather than welcomed. All of that must go some way toward explaining why they have failed to thrive in the economy—and why many young Muslims are poor, disaffected, and unemployed.

At the same time, though, Britain's Muslim immigrants have shown little zeal to become British—certainly nothing to compare with the zeal, over the years, of America's immigrants to become American. To some degree, they have chosen not just religious separateness, but cultural and social separateness as well. And the country went some way to accommodate this preference.

The world would be a happier place, of course, if all immigrants went to their adopted country under a sense of obligation to conform to the manners and mores of their new home, and in the hope that their children would flourish as fully integrated citizens. It would also be good, one can safely say, if the people already living in those countries received the newcomers hospitably, without prejudice or resentment, ready to grant them equal opportunity. America has done pretty well on both counts. Europe has fallen very far short of that ideal. It cannot start over. Now it must live with the consequences.

Starting from here, it is hard to say what the best way forward may be. In education, for instance, France and Britain have taken markedly opposing views. France, militantly secularist, has lately gone so far as to forbid Muslim girls to wear headscarves in schools. Britain, traditionally more tolerant, takes a multiculturalist view, honoring (some say pandering to) religious diversity. But has it made any difference? Both countries have Muslim minorities with plenty of angry, disaffected youth. Will France's hardening approach in schools lessen separateness and alienation among the country's Muslim young, or only make them worse?

Depending on what else happens, it could go either way. Still, my own preference, thinking partly of the harm that church schools did in Northern Ireland, inclines to the French approach. Keep religion a private affair. In schools, "citizenship first, second, and third" seems the better answer.

Britain and France, meanwhile, have to face the present reality of their unassimilated Muslims—as well as the war that fanatical Islamists are prosecuting against the West. For Britain, the ideal bargain on immigration still seems the best guide. In that spirit, Blair's government must recognize that Muslims are sensitive to policies that look like an attack on their religion. This is one more reason why the government must be sure that its new policies really will improve security for all citizens, and explain why. If such measures are only gestures to appease majority opinion (the deportation plan smacks of that), they are wrong on the merits, and divisive as well.

And the other side of the bargain? That is simple. Britain's Muslims need to denounce the terrorists with more rage and no equivocation. To put it bluntly, this is a war, and they need to make it plain—plainer than they have so far—whose side they are on.